It already kind of is 8 core.
The i7 is a 4 core CPU, each core of which contains 2 execution units, so it can behave like an 8 core cpu. If you pull up activity monitor, you see 8 bars in the CPU meter.
The trouble with 8 cores is that most apps will never use that many. You have to be doing something like video encoding to begin to appreciate them - in regular day to day processing you won't see much of a speed up.